Queue qsize = unreliable?

[James R. Saker Jr.]

I see per pydoc that Queue.Queue()'s .qsize is allegedly unreliable:

     |  qsize(self)
     |      Return the approximate size of the queue (not reliable!).

Any thoughts on why this is unreliable (and more curiously, why it would
be put in there as an unreliable function?) Rather than roll my own
threaded fifo class, it would seem prudent to use Python's built-in
Queue but the warning signs on a rather necessary function seem curious.

Jamie